1. Field of the Invention
The present invention relates to an ATM (asynchronous transfer mode) exchange system, and more particularly to an ATM exchange system capable of accommodating a STM (synchronous transfer mode) communication terminal.
2. Description of the Related Art
As a conventional ATM exchange system, for instance, one shown in FIG. 20 is known. This ATM exchange system includes extension ATM terminal interfaces 211-21n to which extension ATM terminals 231-23n are respectively connected via extension cables 221-22n, and ATM trunk interfaces 241-24m to which ATM lines (external lines) 251-25m are respectively connected. It further includes an ATM channel switch 202 to which the extension ATM terminal interface 211-21n and the ATM trunk interfaces 241-24m are connected, and a call control unit 201 which controls the overall apparatus.
If all of the terminal equipments and lines are of the ATM system described above, the configuration shown in FIG. 20 is sufficient. However, if some terminal equipments are of the conventionally used STM system, it is necessary to provide STM/ATM converting units 361-36n having an adaptation function and a cell conversion function between the extension ATM terminal interfaces 311-31n and the extension STM terminals as shown in FIG. 21.
In FIG. 21, the STM/ATM converting unit 361 is disposed in the exchange, while the STM/ATM converting unit 36n is provided outside of the exchange as in the case of a terminal adapter (AT) in an ISDN.
However, with the above-described conventional ATM exchange system, if an attempt is made to accommodate terminal equipment of the STM system in an ATM exchange, the STM/ATM converting unit is required for each terminal. In the case of STM telephone communication, since the internal switch is of ATM system, all the terminals must be firstly converted to be of ATM system and then converted to be of STM system. Accordingly, the cost of the system becomes higher and no benefit can be found in applying ATM system compared with STM system.
In addition, in a case where a terminal is used to serve multiple media, it is necessary to change the cell converting function, such as an ATM adaptation layer format, in the STM/ATM converting unit for each call in correspondence with the attribute of each medium. However, if each STM/ATM converting unit is provided with all the cell converting functions in correspondence with all the media attributes, the STM/ATM converting unit becomes an extremely large circuit. This also contributes to increase the cost.